The connection between obesity and breast cancer risk in women after menopause has long been suspected. Specifically, weight gain from early adulthood into the 60s has been consistently associated with risk of breast cancer after menopause. Cancer researchers believe the reason for this is that fat tissue becomes a major source of estrogen in postmenopausal women, and this estrogen causes certain types of tumors in the breast to grow. Because obese women have more fat tissue, they make more estrogen when compared with women who are thin.

Now, this new study shows for the first time that weight loss directly lowers hormones linked to breast cancer.

Dr. Anne McTiernan, director of the Prevention Center at Fred Hutchinson in Seattle, Wash., and author of the study, said that postmenopausal women who reduce their weight moderately through diet and exercise can lower the amount of these hormones circulating through their bodies, which can in turn decrease their risk of developing breast cancer.

Up to 75 percent of postmenopausal women with breast cancer have the estrogen receptor positive variety, meaning that these cancer cells will grow when estrogen is present. McTiernan estimated that reducing these estrogen levels through weight loss can lower a woman's chance of estrogen sensitive breast cancer by as much as 50 percent.

"Twenty-five to 50 percent breast cancer reduction is estimated based on how much we know estrogen can affect breast cancer risk," she said. "There were nine studies who had been done that showed women with the highest estrogen / testosterone levels had at least a two times increased risk of breast cancer. We estimated that we could see that reduction based on these studies."

Importantly, the study found that even modest weight loss can lower breast cancer risk.

"One main point is that women don't have to be like the 'Biggest Loser,'" McTiernan said. "A lot of people are thinking for general health benefits that they have to lose 50 pounds if they are 200 pounds. That's not what we are seeing.

"Having a first goal of 10 percent of weight lost can have major health effects; it's not as difficult as people are thinking it is."

One of the world's leading epidemiologists, Dr. Walter Willett of Harvard University said the findings were supported by past research.

"From many studies, we know that lower levels [of sex hormones] reduce risk of breast cancer," Willett said. "We have seen that levels of estrogens are about three times higher in obese compared to lean women.

"Weight loss by postmenopausal women is one of the best ways to reduce risk of breast cancer."

Willett also mentions a study showing that women who lost a moderate amount of weight had a 60 percent lower risk of breast cancer.

"And best of all are the side effects: lower risk of diabetes, heart disease, stroke, and other forms of cancer," he said.

While other experts agree weight loss is important they note that there is limited evidence to support these findings.

There is "no direct evidence for this at present," said Dr. Clifford A. Hudis of Memorial Sloan-Kettering Cancer Center in New York.

But, he said, "There is no argument in favor of obesity. Protection from breast cancer is simply one more good reason to be thin, whether it actually prevents breast (or other) cancers needs to be confirmed."